In a cellular communication system, the air interface used for communications from a base station to mobile stations (i.e., the forward link) is typically divided into a number of channels, including traffic channels used to carry bearer traffic (e.g., voice or other user data) and control channels used to carry overhead messages. Depending on the wireless technology used, the air interface can be divided into these channels through code division multiplexing (with each channel defined by modulation with a specific code), time division multiplexing (with each channel defined as a segment of time), frequency division multiplexing (with each channel defined by modulation with a specific frequency), and/or in some other manner.
In traditional systems operating according to the well known Code Division Multiple Access (CDMA) protocol such as CDMA2000, for instance, the forward link may define up to 64 channels, each distinguished by a unique “Walsh code.” The control channels include a pilot channel defined by Walsh code 0, a synch channel defined by Walsh code 32, and a number of paging channels defined by Walsh codes 1 through 7, as necessary. The traffic channels, in turn, are defined by the remaining Walsh codes (up to 62 in total). Further, in a CDMA system, each sector of a base station cell is distinguished by a PN offset, which defines a sector-specific part of a pseudo-random number. Communications between a base station and a mobile station on a given channel, in a given sector, and on a given carrier frequency, are encoded using the Walsh code of the channel and the PN offset of the sector and are then carried on the carrier frequency. A mobile station receiving such a communication can then extract particular channels from the air interface by employing a “rake receiver” that scans through air interface signals in search of a signals that are encoded with particular combinations of PN offset and Walsh code.
Under CDMA2000, each paging channel is divided into a number of timeslots and is used to page a mobile station in order to determine whether the mobile station is available to receive a call or other communication (e.g., incoming packet data). Further the paging channel(s) may carry system information and call setup orders to facilitate establishment of calls or other communication sessions with the mobile station. For instance, the base station may transmit over a paging channel a general page message directed to a particular mobile station, the mobile station may respond to the page message, and the base station may then send to that mobile station over the paging channel a traffic channel assignment message directing the mobile station to tune to a particular traffic channel.
In more advanced systems operating according to the well known EV-DO protocol (as defined by industry standard IS-856 for instance), the forward link is divided into timeslots of 1.67 ms each, and each timeslot is further time division multiplexed to define various channels including a data channel and a control channel. The data channel is used to carry bearer data to a mobile station, and the control channel is used to carry control messages such as page messages for instance. In addition, as with legacy CDMA systems, each cell sector defined under IS-856 may have a respective PN offset and may operate on a particular carrier frequency, and so forward link communications may be encoded using the PN offset, modulated on the carrier frequency, and carried in a particular timeslot.
With EV-DO, the control channel can be used to page a mobile station and to assign traffic channel resources to the mobile station. For instance, the base station may transmit on the control channel a page message directed to a particular mobile station, the mobile station may respond to the page message, and the base station may then send to that mobile station on the control channel a traffic channel assignment message providing identifying information for a particular traffic channel. Still other paging mechanisms may be employed under other wireless communication protocols.
Many of today's mobile stations are highly portable devices running on battery power. If such a mobile station were to constantly monitor the air interface for page messages, the mobile station would quickly run out of battery power. To help avoid this, mobile stations are instead typically arranged to check for page messages only periodically. In particular, when a mobile station is not actively engaged in a communication, the mobile station may remain dormant or asleep but may wake up periodically to check the air interface for a relevant page message. Absent receipt of a relevant page message, the mobile station may then go back to sleep.
In practice, each mobile station may be arranged to operate at a given paging slot frequency that indicates how often the mobile station should wake up to check for page messages. Under CDMA2000 and EV-DO, for instance, each mobile station may have a given “slot cycle index” (SCI), which defines the frequency at which the mobile station will wake up to check the paging channel or control channel for a page. Under existing versions of these protocols, the slot cycle index defines the wakeup frequency by multiplying 1.28 seconds by 2 to the power of the slot cycle index. Thus, a mobile station operating at slot cycle index 0 (zero) would wake up and check for a page message every 1.28 seconds, whereas a mobile station operating at slot cycle index 2 would wake up and check for a page message every 5.12 seconds.
When a mobile station first registers with a cellular serving system, the mobile station and the serving system may negotiate for use of a given slot cycle index, or one may be preset for the mobile station. Thereafter, the serving system may then page the mobile station on a timeslot that the mobile station is set to check. For instance, if a mobile station is operating at slot cycle index 0, then the base station may page the mobile station in a timeslot that is some multiple of 1.28 seconds from time t=0, whereas if the mobile station is operating at slot cycle index 2, then the base station may page the mobile station in a timeslot that is some multiple of 5.12 seconds from time t=0.
Generally speaking, however, the more often a mobile station wakes up to check for a page message, the more quickly the mobile station's battery will drain. Therefore, it is generally desirable for a mobile station to operate at a relatively slow (or infrequent) slot cycle, such as at slot cycle index 2 for instance. Slot cycle index 2 is generally adequate to support paging for incoming telephone calls.
However, in some instances, it makes sense for a mobile station to operate at a faster slot cycle, such as slot cycle 0. By way of example, if a mobile station is operating in a push-to-talk (PTT) mode, in which another user might seek to establish “instant” communication with the mobile station, it would be best for the mobile station to operate at a faster slot cycle, so that the mobile station can more quickly detect a page message and thereby reduce latency in setting up the communication. Still other reasons may justify allowing a mobile station to operate at a higher paging slot cycle (lower slot cycle index). For instance, if the mobile station is actually a fixed position device connected to an AC power source, it might make sense to operate the mobile station at a higher paging slot cycle, since there is no concern that the mobile station's battery would drain from checking for page messages to often.